Validators should be able to determine if URI is preValidated

[satello]

URI's from protocols such as IPFS guarantee file integrity (if fetched from a trusted gateway). Therefore Archon does not need to re-validate these file. At the moment the higher level functions (e.g. getEvidence) tell the utility functions whether or not the URI needs to be validated. validateFileFromURI should be able to handle this on it's own, so that ipfs links can be seamlessly fetched in the same workflow as classic links.

• IPFS gateways are passed to the arbitrator and arbitrable classes so utils do not have access to them.
• This would not solve the case where a URI is passed that already includes the gateway. The uri would have to follow the format /ipfs/... in order to be marked as prevalidated, since not all gateways are trusted by the user
• Possibly add the ability to revalidate ipfs links? It is a bit tricky since ipfs by default does file transformations before hashing, so that would have to be replicated. And it can be opted out of in some cases so it would be difficult for Archon to know the format.